PKeyRSAConverter: RSAKeyValue .NET XML to RSA PEM (and RSA PEM to RSAKeyValue .NET XML)

pkey_rsa_converter.rb

require 'openssl'
require 'base64'
require 'rexml/document'

class PKeyRSAConverter
  def initialize(from_pem: nil, from_xml: nil)
    @from_pem = from_pem
    @from_xml = from_xml
  end

  def to_xml
    xml  = '<RSAKeyValue>'
    xml += "<Modulus>#{xml_base64(xml_pkey.n)}</Modulus>"
    xml += "<Exponent>#{xml_base64(xml_pkey.e)}</Exponent>"
    xml += '</RSAKeyValue>'
    xml
  end

  def to_pem
    pem_key.to_pem
  end

  private

  def pem_key
    exponent = xml_find_exponent.to_s
    modulus = xml_find_modulus.to_s
    key = OpenSSL::PKey::RSA.new
    key.set_key(pem_base64(modulus), pem_base64(exponent), nil)
    key
  end

  def xml_find_modulus
    REXML::XPath.match(xml_document.root, '//RSAKeyValue/Modulus/text()')[0]
  end

  def xml_find_exponent
    REXML::XPath.match(xml_document.root, '//RSAKeyValue/Exponent/text()')[0]
  end

  def xml_document
    @xml_document ||= REXML::Document.new(@from_xml.to_s)
  end

  def pem_base64(string)
    Base64.urlsafe_decode64(string).bytes.inject(0) { |a, e| (a << 8) | e }
  end

  def xml_pkey
    @xml_pkey ||= OpenSSL::PKey::RSA.new(@from_pem)
  end

  def xml_base64(int)
    Base64.encode64([int.to_s(16).downcase].pack('H*')).split("\n").join
  end
end

# Example

rsa_xml = '<RSAKeyValue><Modulus>lg6UHw38R0kv2eoSxE0X42j/1CSY9pf/HGk+V5lNM2BqRORSWbJk+53x4QaxpfucuChUXf3NOtd6DiEh5pawCw==</Modulus><Exponent>AQAB</Exponent></RSAKeyValue>'
rsa_pem = "-----BEGIN PUBLIC KEY-----\nMFwwDQYJKoZIhvcNAQEBBQADSwAwSAJBAJYOlB8N/EdJL9nqEsRNF+No/9QkmPaX\n/xxpPleZTTNgakTkUlmyZPud8eEGsaX7nLgoVF39zTrXeg4hIeaWsAsCAwEAAQ==\n-----END PUBLIC KEY-----\n"

puts 'Convert PEM to XML'
to_xml = PKeyRSAConverter.new(from_pem: rsa_pem).to_xml
puts to_xml == rsa_xml

puts 'Convert XML to PEM'
to_pem = PKeyRSAConverter.new(from_xml: rsa_xml).to_pem
puts rsa_pem == to_pem

I have no idea why I did this @xiaoronglv, but thanks !
Code updated.